1. Field of the Invention
The present invention relates to photographic materials which comprise, in combination, a precursor of a photographic reagent capable of releasing a photographically useful reagent timely on photographic processing and a silver halide emulsion layer.
2. Description of the Prior Art
When photographically useful reagents are added to a photographic material, several advantages are attained due to the mixture of the reagents, which are totally different from the use of photographic reagents contained in a processing solution. For example, (1) it is possible to use photographic reagents which are able to readily decompose with acids or alkalis or under oxidation or reduction conditions and which cannot withstand a processing bath for a long period of time, (2) the composition of a processing solution is simple and is easy to prepare, (3) it is possible to permit a necessary photographic reagent to react with good timing at the time of the processing, (4) it is possible to allow a necessary reagent to act on a necessary spot, i.e. a specific type of layer and/or its adjacent layer alone of a multi-layer photographic material, and (5) the amount of a photographic reagent can be varied as a function of development with silver halide. However, if photographic reagents are added to a photographic material in the active form, they react with other ingredients of the photographic material during storage prior to the processing or decompose by the action of heat or oxygen. Thus, they cannot exhibit the performance as will be expected at the time of the processing.
One of the processes known in the art to solve the above problem is to block active groups of a photographic reagent into substantially inert groups in the photographic material. In other words, the photographic reagent is added to the photographic material as a precursor of a photographic reagent reacting only at the time of the developing process.
According to the above process, when the useful photographic reagent is, for example, a dye, functional groups which influence the spectral absorption of the dye are blocked and cause the spectral absorption to be shifted toward a side of a shorter or longer wavelength. Accordingly, if such a dye coexists in a silver halide emulsion layer having a desired light-sensitive spectral range, any lowering of the sensitivity due to a filler effect does not occur. When the useful photographic reagent is an antifoggant or a development restrainer, blocking of the active groups permits the desensitizing action due to absorption on the photosensitive silver halide or its formation of a silver salt during storage to be suppressed. At the same time the fogging is reduced without deteriorating the sensitivity, the fogging by excess development can be suppressed or the development can be stopped at any time required timely release of the photographic reagent during the developing process. When the useful photographic reagent is a developing agent, a developer aid, a development accelerator or a nucleating agent, various adverse photographic effects the formation of semiquinone or oxidants by oxidation with air during storage or formation of fogging nuclei during storage caused by injection of electrons into the silver halide can be prevented by the blocking of the active or adsorbing groups. As a result, stable photographic processing is ensured. Similarly, when the photographic reagent is a bleaching promotor or a bleaching and fixing promotor, blocking of the active groups permits the reaction with other ingredients during storage to be suppressed so that a required performance may be obtained at a required time by removing protecting groups at the time of the development.
As explained hereinabove, the precursors of photographic reagents are effective in producing a satisfactory performance of the photographic reagents. On the other hand, however, these precursors must satisfy very severe reciprocal requirements. The precursors must meet such a contradictory requirement that they are stable under conditions of storage and the blocking groups can be removed at a required time so as to rapidly and efficiently release the photographic reagent.
Several blocking techniques for photographic reagents are known in the art. For instance, Japanese Patent Publication No. 44805/72 describes the utilization of blocking groups such as acyl, sulfonyl and the like groups. In Japanese Patent Publication Nos. 39727/79, 9696/80 and 34927/80, blocking groups which can release a photographic reagent according to a so-called Michael reaction are utilized. Japanese Patent Publication No. 39727/79, and Japanese Patent Application (OPI) Nos. 135944/82, 135945/82, and 136640/82 describe the utilization of blocking groups capable of releasing photographic reagents accompanied by the formation of quinone methide or its analogous compounds by the electron transfer in the molecule. In Japanese Patent Application (OPI) No. 53330/80, there is described the utilization of an intramolecular ring closure reaction. Japanese Patent Application (OPI) Nos. 76541/82, 135949/82 and 179842/82 describe the utilization of cleavage of five-membered or six-membered ring compounds.
These known blocking techniques for photographic reagents are disadvantageous in that, when using stable precursors under storage conditions, the speed of releasing a photographic reagent at the time of the processing is so small that the precursors have to be subjected to a high alkali treatment at a pH over 12. Even though a satisfactory releasing speed is satisfied by the use of a processing solution having a pH of 9 to 12, precursors gradually decompose under storing conditions and lose their characteristic properties.
When a liquid developer having a high pH over 12 is used, the developer is susceptible to oxidation with air. Thus, the shelf stability of the developer becomes lower and corrosion of a developing bath is accelerated. In addition, because of the high skin irritativeness, it becomes difficult to handle. Furthermore, the photographic sensitivity and the stability of a formed image are difficult to maintain when using a liquid developer of a higher pH.